The present invention pertains generally to optical transmission systems. More particularly, the present invention pertains to devices and methods for optically linearizing the nonlinear distortions that are inherently introduced into signals produced within the optical transmission system. The present invention is particularly, but not exclusively, useful as a signal processor that can be incorporated into a fiber optic transmission system to optically compensate for the second and/or third order distortions that are produced in the system.
In a conventional fiber optic transmission system, an electrical information signal is used to modulate the intensity of an optical transmitter. The resultant modulated signal is then transmitted over a distance through an optical fiber. After passing through the optical fiber, the modulated signal is converted back into an electrical signal by an optical receiver. It is well known that the information carried on such a modulated signal may be either in a digital, an analog or a mixed signal format. For several reasons, such as an enhanced multiplexing capability, there is an increasing interest in delivering digital information via optical fibers in an analog format. Fiber optic transmission systems, however, are susceptible to degrading distortions which can significantly affect the quality of the communications.
Laser diodes are well known devices that are now commonly used for transmitting signals with an analog format over a fiber optic transmission system. Laser diodes, however, like all other analog optical transmitters, have nonlinear transmission responses. Unfortunately, the nonlinearites introduced by the transmitters are often aggravated by the optical fiber, or by certain other optical components in the system. It happens that these nonlinear responses are predominantly second-order distortions and third-order distortions (distortions that can create spurious frequencies that fall within the bandwidth of interest). Obviously, it is desirable that these distortions be removed from transmitted signals.
Heretofore, several electronic methods have been disclosed for the purpose of linearizing the outputs of laser diode transmitters. More specifically, these methods have been directed toward linearizing laser diodes when they are used in fiber optic analog transmission systems. In general, these methods have tended to include the use of predistortion circuits that intentionally generate distorted signals having second-order or third-order distortions. Typically, the distorted signals are generated with the same amplitude as the distortions generated in the system, but they have an opposite phase. Thus, when added together, the distortions generated by the predistortion circuits are intended to cancel the distortions that are introduced into the system by the laser diode transmitter. Predistortion circuits in general are limited in bandwidth due to the state of the art electronic circuit limitation.
In addition to predistortion circuits, it is known that other linearization schemes can be fabricated in several ways. For example, known linearization schemes include: 1) cancellation by complimentary outputs; 2) push-pull operations with wavelength division multiplexing; and 3) the use of Fabry-Perot devices. These linearization schemes have, however, been susceptible to system degradation for several reasons. Specifically, scheme 1, as mentioned above, suffers from the need for two stable transmission fibers and scheme 2 suffers from electronic component bandwidth limitation. Furthermore, scheme 3 suffers from instability due to the difficulty in maintaining precise wavelength alignment between the laser diode and the Fabry-Perot.
In light of the above, it is an object of the present invention to provide an optical apparatus for linearizing the output of an optical transmitter. Another object of the present invention is to provide an apparatus for linearizing the output of an optical transmitter that can be effectively wavelength (e.g. temperature) tuned for proper operation. Yet another object of the present invention is to provide an apparatus for linearizing an optical transmission system with selected optical devices that can have either linear or nonlinear wavelength dependent optical responses. Still another object of the present invention is to provide an apparatus for linearizing an optical transmission system that is simple to use, is relatively easy to manufacture, and is cost effective.
In accordance with the present invention, a communications apparatus for linearizing the output of an optical transmitter (such as a DFB laser diode), includes an optical device (such as a fused fiber WDM coupler). Specifically, the optical device is connected to receive the output of the optical transmitter. It happens that the output from the optical transmitter will include a modulated signal, as well as second and third order distortions (hereinafter sometimes collectively referred to as a xe2x80x9ctransmitter distortionxe2x80x9d). Importantly, the transmitter output also includes a characteristic wavelength xe2x80x9cchirpingxe2x80x9d. In accordance with the present invention, this xe2x80x9cchirping,xe2x80x9d together with the desired transmitter output, is used as an input by the optical device, to optically generate nonlinear distortion signals (hereinafter sometimes collectively referred to as xe2x80x9ccompensation distortionsxe2x80x9d) that will compensate the transmitter distortion. Accordingly, the compensation distortions can be added to the output of the optical transmitter to cancel the transmitter distortions (second and/or third order distortions) in the output.
Technically, the modulated signal that is transmitted by the transmitter (e.g. laser diode) will have a center emission wavelength (xcexc) and a characteristic wavelength chirping (dxcexc). Further, the optical device (e.g. coupler) will include components for establishing a predetermined, wavelength dependent, normalized optical transfer curve F(xcex). Specifically, this optical transfer curve F(xcex) is fabricated to accommodate the operating condition of the optical transmitter. In particular, the optical transfer curve F(xcex) of the optical device is designed to have a reference wavelength (xcexp), a slope determinant wavelength spacing (xcex94xcexw), and an operating point wavelength offset (xcex94xcexb) that are all based on the known operating conditions of the transmitter.
In their connection with each other, the optical device and the optical transmitter can be individually or collectively wavelength (e.g. temperature) tuned. Preferably, an operating temperature for the optical transmitter (or optical device) can be established which will align (xcexc) of the transmitter with (xcexp+xcex94xcexb) of the optical device. Regardless how the operating temperature is established, when the system is tuned, an operating point can be established on the optical transfer curve F(xcex) that will interact with the wavelength chirping (dxcexc) from the transmitter in a specified manner. Preferably, this operating point is established on the optical transfer curve F(xcex) where xcexp+xcex94xcexb=xcexc. Thus, the purpose here is to use F(xcex) to optically induce a compensation distortion from the wavelength chirping (dxcexc) that will substantially compensate the transmitter distortions (second and/or third order distortions) that are introduced by the transmitter. Once the compensation distortions have been induced by the optical device (e.g. coupler), linearization of the optical transmitter (e.g. laser diode) is accomplished by adding the compensation distortion to the output of the transmitter. Stated differently, the compensation distortion is added to the output of the optical transmitter to cancel the transmitter distortion from the modulated signal in the output.